Fiber-reinforced composite materials containing an epoxy resin composition serving as a matrix resin have been widely used for aircrafts, automobiles, and industrial applications due to their superior mechanical properties and the like. Especially, as for structural and interior materials for aircrafts, from the perspective of achieving lighter weight, there are more cases where fiber-reinforced composite materials are used as face sheets of honeycomb panels.
Conventionally, a honeycomb panel is produced in so-called co-cure molding in which a film-type adhesive agent is placed on both sides of a honeycomb core, a prepreg (uncured fiber-reinforced composite resin material) serving as a face sheet is laminated on the both sides of the honeycomb core, and curing of a resin constituting the prepreg and adhesion between the face sheet and the honeycomb core are performed simultaneously. However, in order to obtain a lighter honeycomb panel and to reduce cost for molding, a so-called self-adhesion technique for directly bonding a honeycomb core and a prepreg has been demanded. However, since self-adhesiveness of a prepreg is associated with viscosity and toughness of resin, depending on how they are adjusted, porosity (uneven part on the surface) in the prepreg laminated on a honeycomb core may be caused, or tackiness and drapability of the prepreg may be impaired, resulting in lowering productivity. Therefore, it has been an extremely difficult task to increase the self-adhesiveness.
For improving self-adhesiveness of a prepreg, it is important to form a fillet by wetting a bonding surface between a honeycomb core and the prepreg with a prepreg resin during thermal curing, so-called fillet forming, and to obtain a favorable shape and strength of the fillet. The fillet is formed by the resin in a state or hanging or climbing from the prepreg in a thickness direction of the honeycomb core along a wall of the honeycomb. The shape of the fillet is deeply associated with the viscosity of the resin. If the degree of the hanging or climbing is too large, porosity is more likely to be formed, resulting in an uneven part on the surface. It the viscosity is too high, tackiness (adhesiveness) and drapability (flexibility) are impaired. In addition, the strength of the fillet is affected by the toughness of a resin constituting the prepreg.
Patent Document 1 proposes a self-adhesive prepreg having a matrix resin composition including a thermosetting resin, a viscosity control agent made of a thermoplastic resin which dissolves in the thermosetting resin, thermoplastic resin particles which are not dissolved in the thermosetting resin, and a curing agent. Likewise, Patent Document 2 proposes a woven fabric prepreg having a matrix resin composition including thermoplastic resin particles and a thermosetting resin composite containing a thermoplastic resin. However, in these self-adhesive prepregs, although the viscosity characteristics of a resin composition is improved and the shape of fillet is modified, there are a problem that tackiness and drapability, which are important characteristics of workability in prepreg handling, are poor, and a problem that the prepregs are inferior in porosity. Therefore, the proposed self-adhesive prepregs above have a problem that the workability and appearance quality thereof are not at a sufficient level although the adhesiveness thereof has been improved to some extent.    Patent Document 1: Japanese Patent Translation Publication No. 2005-506394    Patent Document 2: Japanese Patent No. 3661194